Windows opening on wrong monitor (e.g., Vivaldi Settings)

This probably isn't the right place to ask a technical question like this, but it is somewhat Vivaldi-related, so I'll try here first. I'm basically a Linux noob. I've figured out some stuff, but I've mostly used Windows. I'm currently running Mint 18 as a dual-boot. I have a dual-monitor set up, with the monitor on the right side set as the primary monitor. I often leave my left monitor turned off unless I need it. Most windows open on the right (primary) monitor. Many programs do not - they open on the left monitor. Vivaldi is one example. If I run Vivaldi, the browser opens on the right (primary) monitor. However, when I open the 'Settings' as a window, the Settings window opens on the left (secondary) monitor. I found this link: http://segfault.linuxmint.com/2015/08/configuring-the-mdm-login-screen-for-multiple-monitors/ which suggests using 'mdmsetup' to select a Primary monitor. When I try, the Primary monitor dropdown box has no options, it's completely blank. That page has additional suggestions, but I was hesitant to try at this point. I wanted to ask a community like this first, since many times when I have Linux issues, I see a bunch of command lines suggested to fix the problem (and often different people suggesting different command lines to accomplish the same thing) but I don't really learn anything from it. Any suggestions? And why would the 'Settings' window in Vivaldi not respect the primary monitor, while the browser itself does?